In rivalry with the silicon transistor based on the crystalline silicon technology, the development of the transistors utilizing organic semiconductors has been actively carried out. Organic semiconductors are provided with the characteristics inherent to organic materials including lightweightness, flexibility, diversity, and toughness; organic semiconductors can be formed by the low temperature processes around 100° C., and permit the fabrication method based on the liquid phase processes such as printing and spin coating. Accordingly, the fabrication on the plastic substrates and the enlargement of the image planes, both having been impossible with the crystalline silicon semiconductor, can be possible, and thus, more expectations are placed on the application of organic semiconductors to novel devices such as flexible electronic paper and information tags.
However, the usual organic semiconductors are as low as 10−5 to 10−2 cm2/Vs in mobility, lower in mobility by an order of magnitude or more than the silicon semiconductors, and thus higher in resistance, so that the organic semiconductors face the problems such that large currents can be hardly obtained and the operation frequencies are low.
The mobility that can be obtained with a field effect organic transistor can be derived from the formula (I) of the drain current in the saturation region.Id=μ(W/2L)Ci(Vg−Vth)2  (I)In this formula, Id denotes the drain current (A), μ denotes the mobility (cm2/Vs), W denotes the channel width (cm), L denotes the channel length (cm), Ci denotes the capacity of the gate insulating layer (F/cm2), Vg denotes the gate voltage (V), Vth denotes the threshold voltage (V) of the transistor, which can be obtained from the relation between the square root of the drain current and the gate voltage by extrapolation to the drain current Id=0.
Additionally, when used as a switching device, it is necessary that the ratio (on/off ratio) of the current flowing between the source and drain electrodes when the transistor is on to the current flowing between the source drain electrodes when the transistor is off be at least 104 or more, preferably 106 or more. However, in the case of an organic semiconductor, the mobility is low as described above, and hence the on current is small; on the other hand, owing to the effect of the impurities contained in the organic semiconductor, the off current is large; so that a sufficiently large on/off ratio has not yet been obtained. This is conspicuous for polymer compounds.
A technique for improving the on/off ratio, disclosed in Japanese Patent Application Laid-Open No. 10-190001, has made the mobility be 10−3 cm2/Vs or more and the conductivity be 10−5 S/cm or less, and thus has made the on/off ratio be of the order of 103; however, this is not yet sufficient. Thus, it is the actual condition that there is no field effect organic transistor which utilizes an organic semiconductor capable of satisfying the practical characteristics.